Low frequency heater



FIG. 1 is a front elevation view of a low frequency heater showing my new design;

FIG. 2 is a rear elevation view thereof;

FIG. 3 is a top plan view thereof;

FIG. 4 is a bottom plan view thereof;

FIG. 5 is a right side elevation view thereof; and,

FIG. 6 is a left side elevation view thereof. 

The ornamental design for a low frequency heater, as shown. 